Elders with mild Dementia exhibit impairments of memory, thought and reasoning. It has been recognized that pervasive computing technologies can assist those suffering from mild Dementia to improve their level of independence and quality of life through cognitive reinforcement. In this paper, we present a user-centred design approach, which is based on the iterative process of user study, prototyping, user test and evaluation, to achieve the goal of developing a cost-effective cognitive prosthetic device with associated services for elders with mild Dementia. Specifically, we describe the results of user study in three different test sites, four areas of cognitive reinforcement have been identified to assist their independent living. Of different assistive services, we choose two context-aware reminding services as a case study to illustrate how to deploy pervasive computing techniques in the system design. Finally, we present the overall system architecture and initial system implementation with first trial results